The invention relates to rotary drill bits for use in drilling or coring holes in subsurface formations and, in particular, to cutter assemblies and cutting elements for use on such bits, and methods of manufacturing such cutting elements
Rotary drill bits of the kind to which the invention relates comprise a bit body having a shank, for connection to a drill string, and a passage for supplying drilling fluid to the face of the bit. The bit body carries a plurality of so called "preform" cutting elements at the surface. Each cutting element comprises a thin facing table of superhard material, which defines the front cutting face of the element, bonded to a less hard substrate.
Normally the superhard table is formed of polycrystalline diamond material and the substrate is formed of cemented tungsten carbide. The preform cutting elements are usually mounted on the bit body by each being bonded to a carrier which may be in the form of a stud of cemented tungsten carbide which is received and located in a socket in the bit body. The cutting elements may be bonded to their respective carriers by brazing, for example using the process known as "LS bonding".
Tungsten carbide has certain characteristics which render it particularly suitable for use in the substrate and carrier of such a cutter assembly. Thus, it exhibits high rigidity, high resistance to the erosion to which such cutter assemblies are subject in use, and hot strength. Also, the coefficient of expansion of tungsten carbide is sufficiently close to the coefficient of expansion of polycrystalline diamond to reduce the residual stresses which can occur when the two materials are bonded together. However, some of the other characteristics of cemented tungsten carbide have certain disadvantages. For example, cemented tungsten carbide has low toughness (i.e. it is comparatively brittle) and this can lead to failure of such cutter assemblies in use, as a result of impact forces on the assembly.
Also, when a preformed cutting element of the kind referred to is mounted on a drill bit, it is mounted with its front face facing in the direction of movement, its rear face trailing behind and part of the peripheral surface of the cutting element, between the front and rear surfaces, rubbing on the formation being drilled. As drilling proceeds this intermediate surface wears down to form a wear-flat. After prolonged use, this flat may extend on to the carrier.
Due to its two-layer construction, the cutting element is self-sharpening since the part of the less hard substrate which rubs on the formation wears away more quickly than the part of the diamond table rubbing on the formation. Since the rearward substrate wears away more quickly, it does not bear on the formation being cut to the same extent or with the same pressure as the diamond table. The compact is therefore rendered self-sharpening by this constant greater wearing away of material rearwardly of the front cutting face.
However, the abrasion resistance of the wear-flat rubbing on the formation generates heat which is additional to the heat generated by cutting. Tungsten carbide has high abrasion resistance, and when the substrate or carrier is formed from tungsten carbide the additional heat generated by abrasion may be sufficient to cause the thermally activated deterioration of the diamond table at an increasingly rapid rate. It is generally accepted that a standard cutting element of the kind described, having a tungsten carbide substrate, generally operates efficiently only until it is about 30% worn. From thereon the cutting element may become thermally unstable and wear extremely rapidly leading quickly to failure of the cutter. When a sufficient number of cutting elements have failed in this manner the bit becomes useless for further drilling.
The combination of low toughness and high heat generation can also cause heat checking of the tungsten carbide carrier material with resultant premature failure of the bit. "Heat checking" is a term in the art which refers to craze cracking of the wear flat which develops on the carrier, due to abrasive heating with intermittent quenching by the drilling fluid.
The approach taken by the present invention is to overcome the above-mentioned problems by reducing the abrasion resistance of the less hard substrate of the cutting element and/or the carrier so as to reduce the additional heat generated by rubbing of the wear-flat on the formation being drilled It is, however, desirable that abrasion resistance of the substrate and/or carrier should be reduced without also reducing its erosion resistance. In use the cutter assemblies and their cutting elements are subjected to the substantial eroding effect of drilling fluid flowing over them continuously during drilling. Cemented tungsten carbide has considerable resistance to such erosion and this should preferably not be significantly reduced in attempting to reduce the abrasion resistance.